Pressure and temperature measurements are two of the most common parameters that need to be measured in nearly every industrial sector. Most of the current pressure sensors and measurement devices are based on the use of semiconductors, such as silicon. However, some pressure measurement needs are difficult to be fulfilled by the existing pressure sensor technologies due to various measurement environment challenges, such as high temperatures, electromagnetic interference (EMI) and remote signal transmission as required in oil/gas downhole measurement.
Single-crystal sapphire has a melting point of 2045° C. and is known to exhibit excellent resistance to chemical corrosion. In addition, it is transparent to a broad range of optical spectrum from ultra-violate (UV) to infrared (IR) and is relatively low cost. Besides the capability of high temperature operation and resistance against chemical corrosion, single-crystal sapphire is also known to offer excellent stability in other harsh environments. For example, many amorphous materials, such as glasses, show creep under high pressure especially at elevated temperatures. Further other foreign chemical species, such as gases and water can gradually diffuse into these materials under high pressure. The diffusion rate will pick up as temperature increases. The sensors built with amorphous materials will therefore exhibit drifts under these operating conditions. In contrast, single-crystal sapphire may exhibit minimal material creep and foreign material diffusion even under high pressure and elevated temperature. Therefore, single-crystal sapphire is an attractive material for construction of sensors for excellent long-term stability under high temperatures or high pressure or both even with presence of other diffusive species, such as various gases and water.
To build a pressure sensor, a hermetically sealed hollow cavity that can change in response to an externally applied pressure may be provided. Construction of such a hollow cavity may include bonding between different mechanical parts. For a sapphire pressure sensor, sapphire-to-sapphire direct bonding may provide benefits. The resulting sensor may offer high long-term stability. Additionally, the sensor may also have ultra-high temperature operation capability.
Two major sapphire-to-sapphire direct bond methods are known. One method was reported by A. Sugiyama, et al. [A. Sugiyama et al., “Direct bonding of Ti:sapphire laser crystals,” Appl. Opt., vol. 37, p 2407, 1998]. This method consists of two steps. The first is to pre-bond two sapphire elements at a temperature around 200° C. The second step is to bake the pre-bonded sapphire assembly at a temperature above 1000° C.
The other method is plasma assisted bonding, described in U.S. Patent Application Publication No. 2012/0024073. Plasma assisted bonding may substantially reduce the baking or anneal temperature. Using a method similar to the one reported by Sugiyama et al., Virginia Tech researchers lately constructed a hermetically sealed sapphire Fabry-Perot (FP) cavity and demonstrated pressure measurement at room temperature [J. Yi, et al., “Demonstration of an all-sapphire Fabry-Perot cavity for pressure sensing,” IEEE Photon. Tech. Lett., vol 23, p 9, 2011]. In this work, two a-cut sapphire wafers were used. One was etched to form an approximately 6 μm circular pit using a reactive ion etching (RIE) process. This etched wafer was then bonded to another wafer based on sapphire to sapphire direct bond. The FP cavity was then glued to a ceramic tube. A multimode fiber was inserted into the ceramic tube to the FP cavity for the sensor interrogation. The FP cavity was demodulated using whitelight interferometry. Because of the shallow FP cavity, a very broadband spectrum halogen lamp was used as the source along with an Ocean Optics spectrometer. In their test, the whole sensor including the FP cavity and the ceramic tube were placed in a pressure chamber and the fiber ran through a fiber feedthrough.
This sensor structure is not ideal for real applications. A practical pressure sensor usually has a metal casing with mechanical threads for convenient pressure-sealed sensor installation to a pressure vessel. Further, due to the significant mismatch in the coefficients of thermal expansion (CTEs) between the sapphire FP cavity and the ceramic tube, this sensor structure may not survive at high temperatures. Also, the thermal stresses induced by the CTE mismatch will introduce significant thermal dependence of the FP cavity distance and this dependence may not be repeatable due to the gradual release of the stresses trapped in the adhesive during its cure.